The quality of air in enclosed spaces such as houses and other buildings is subject in a recently released Environmental Protection Agency Report titled "EPA Report to Congress on Indoor Air Quality", released Aug. 4, 1989. In that report reference is made to the so called "Sick Building Syndrome" and a program of increased research and information dissemination regarding the dangers of poor indoor air quality is recommended. Health effects attributed to air contaminants accumulating in poorly ventilated houses and other buildings range from eye, ear, nose and throat irritation, to full scale respiratory and neurological diseases, genetic mutations and cancer. Contaminants such as radon, asbestos, tobacco smoke, formaldehyde, volatile organic compounds, chlorinated solvants, biological contaminants and pesticides etc., and the synergistic effects of multiple contaminants are cited as causes of health problems.
The report suggests that reducing the sources of contaminants is the most direct and dependable option in overcoming the problem, and that while air cleaning equipment can complement air quality improvement, there is no substitute for providing an adequate ventilating inlet fresh air volume inflow rate into an enclosed space.
In recent years, the high cost of energy has led many people to strive to make their houses and buildings more tightly sealed, hence, in combination with the use of insulation, more energy efficient. Said efforts have included sealing cracks and other air leaks in their houses or buildings to prevent heated or cooled air from escaping, and outside air which requires heating or cooling, from randomly entering at an excessive rate. In effect, such houses and buildings become, to various degrees, closed systems. In such structures the inside air replacement rate is often reduced to far below the American Association of Heating, Refrigeration and Air Conditioning Engineers recommended minimum fresh air inlet volume flow rate of 15 Cubic Feet per Minute (CFM) per inhabitant, or 35% enclosed space air change per hour, whichever is greater, (see ASHRAE Standard 62-1989). The result of an insufficient inlet fresh air volume flow rate into, and out of, such tight enclosed spaces is that contaminants accumulate inside same to dangerous health affecting levels. To emphasise this point, it is estimated by some health care researchers that presently two persons per hour, in the United States alone, contract lung cancer as a result of contact with radon in poorly ventilated houses and other buildings.
It now seems obvious that ventilation should be carefully controlled so that an adequate oxygen supply is assured, contaminants in the air are filtered out, and excess air leakage into and out of enclosed spaces is minimized.
Given that, aside from the potential health hazards, making houses and buildings more energy efficient is desirable, then it follows that a method which would provide a sufficient health maintaining inlet fresh air volume inflow rate ventilation to a tight structure would be of great benefit. A search of existing Patents shows that numerous inventors have realized this and have proposed systems, and methods of their use, which provide controlled ventilation to enclosed spaces such as houses and buildings. The various approaches basically utilize a means to cause air flow, such as a motor driven blower, to cause air to move into and stale air to move out of an enclosed space. The inlet fresh air volume inflow rate is typically, but not necessarily in the most basic schemes, controlled based upon signals developed by sensing air pressure differences between the inside and the outside of a house or building, from signals derived from sensed rates of air flows in various parts of a system, or by sensing the velocity of the wind outside the house or building.
The most basic schemes simply provide a large inlet fresh air volume inflow rate into a house or building sufficient to raise the air pressure inside the house or building to a large positive value with respect to that outside the house or building. In such a scheme the inlet fresh air volume inflow rate must be large enough to maintain the positive pressure difference no matter what active or passive exhaust air flows develop. As an example, operating a cloths dryer or fireplace will actively force exhaust air from a house, and opening a door to the outside of the house or building can passively increase exhaust air. The problem with such simple large positive pressure systems is that they are wasteful of energy. The large volume of air which is flowed into a house or building equipped with such a system must be heated or cooled at times. As very large inlet fresh air volume inflow rates are not necessary to keep contaminant concentration levels low enough for health maintenance reasons, there is no valid reason to provide them to a tight house or building. Inventors have noted this and responded. For instance, Lorenz, in U.S. Pat. No. 3,611,906 and Van Huis in U.S. Pat. No. 4,043,256 teach systems which sense inside and outside air pressure and from same develop signals which are used to control the amount of inlet fresh air volume flow rate through a house or building, based upon the difference in said signals. That is, the flow of air into and out of a house or building is modified as required, by use of inlet air or exhaust air fans, to dynamically keep the inside air pressure above that outside the house or building. The problem with such schemes is that outside air pressure is used as a reference, and because of quickly occurring large magnitude wind induced changes in outside air pressure near houses, buildings and other obstructions, that reference is not particularly constant. A Russian Patent to Slavin et al., No. SU-590-556 teaches a system which goes some distance toward overcoming this defect by sensing wind velocity and combining a wind velocity derived signal with an outside atmospheric pressure derived signal, which combined signal is used as a basis to control inlet fresh air volume inflow rates. The problem remains, however, that wind induced pressures can change very quickly and significantly and control systems tend to become unstable when a reference signal changes quickly and with significant magnitude. A Patent to Johannsen, U.S. Pat. No. 4,257,318 recognizes that a constant reference signal is necessary to assure stability in a control system, and Johannsen focuses on the use of a user set reference signal level to which are compared numerous air pressure representing signals, which air pressure representing signals are produced by sensors in various locations in air ducts in a house or building. The Johannsen approach selects the lowest such sensed air pressure representing signal and that signal is compared to the user set reference signal. Inlet fresh air volume inflow rate is controlled based upon the signal resulting from the comparison. The Johannsen invention also provides for adjustable dead bands in the comparison circuitry to enhance stability. The problem with the Johannsen system is that, just as in the most basic large positive pressure schemes, the selected reference signal has no definite relationship to any relevant reference pressure, hence, the inlet fresh air volume inflow rate can be unknowingly set to energy wasting levels which are higher than necessary to provide a healthy environment inside a house or building, over time. A Patent to Haines et al., U.S. Pat. No. 4,407,185 teaches the sensing of pressure in a plenum system and controlling fresh air volume flow rates so that said pressure is typically maintained at a negative value with respect to outside air pressure. As a result outside air flows into the plenum. The reference signal is, however, derived from outside air pressure by a sensor which is exposed to wind, and thus the reference signal can be rapidly and significantly changed by wind induced pressure fluctuations, as has already been noted. It is added that while retaining a negative pressure in a plenum is an acceptable way to draw air into same, keeping a negative pressure in a house or building, relative to outside air pressure, can lead to, for instance, outside air being drawn into the house or building down through a chimney, thereby blocking the exit of dangerous gasses which result from the burning of fuel. The results can be deadly. A Patent to Dean et al. teaches a system for use in hospitals. A fresh air volume flow rate controlling signal is derived from the difference between air pressure signals derived by sensors located in a hospital room and in the hall outside the hospital room. An air volume flow rate is set, based upon the difference in said signals, which is sufficient to keep a positive or negative pressure in the room with respect to the pressure in the hall. While the pressure in the hall of a hospital will be relatively more constant than that outside the hospital, it will still change when doors are opened or closed etc. The reference pressure is variable and may have frequent fluctuations of a significant magnitude due to wind induced pressures. Also, since air is forced from one portion of the hospital to another, portion of the hospital from which air is removed may develop an air pressure therein lower than soil gas pressure. This can induce soil gas under that portion of the hospital to enter that portion of the hospital and contaminate it.
It will be appreciated that the systems surveyed above provide inlet fresh air volume inflow rates which use reference signals which are simply set arbitrarily, or which are derived based upon references signals which are not relatively constant. As well, the basic approach is to provide inlet fresh air volume inflow rates which are sufficient to keep a significant pressure differential in place. In either case the inlet fresh air volume inflow rates provided will, over time, be in excess of what is actually needed to provide a "just adequate" ventilation, from a health maintenance perspective. A Patent to Wallin, U.S. Pat. No. 4,620,398 suggests a different approach and teaches that the soil gas pressure under a slab upon which is present an enclosed space should be monitored and compared to the air pressure inside the enclosed space. However, the teachings are that the difference in the indentified pressures should be used to control an air pump which forces air into the sub-slab location, thereby sweeping soil gas out from under the slab, but in the process changing the soil gas pressure. In "Radon Reduction Techniques for Detached Houses, Technical Guidance (Second Edition)", by D. Bruce Henschel, EPA Report No. EPA/625/5-87/019, Revised January 1988 it is mentioned, on page 154, that if a method for maintaining a consistent pressurization in the basement of a house, above that of the soil gas pressure, can be derived, it could turn out to be a potentially attractive approach where it can be applied. Reference in said publication is made to the possible use of soil gas as a stable reference, but it is noted that no system and method are disclosed in that publication for making said use of the soil gas pressure as a stable reference pressure.
A need exists for a ventilation control system which identifies and utilizes a relatively constant pressure reference which can be compared with indoor air pressure, so a signal can be derived, so that variation in the signal can be used to control the inlet fresh air volume inflow rate into, and stale air volume flow rate out of, an enclosed space such as a house or building. Additionally, a need exists for a ventilation control system which does not typically maintain an excessive positive or negative air pressure in an enclosed space, or part thereof, and which provides ventilation in the amount which is just necessary to provide an adequate health maintaining, ventilation inlet fresh air volume inflow rate into, and stale air volume flow rate out of, the enclosed space, so that adequate oxygen is supplied and indoor air contaminant concentrations are kept below dangerous levels.